1. Field of the Invention
The present invention relates to a circuit for recovering a signal of interest which is offset by a common mode displacement and, more specifically, to a circuit for converting an analog current sense signal in a motor drive controller circuit from a high side reference potential to a low side reference potential for measurement and processing.
2. Description of the Related Art
In many applications, an analog signal of interest may be inherently offset by a common mode potential between the signal itself and a circuit intended to condition, measure or process that signal in some way. Such is the case, for example, in a motor controller circuit, where current to the motor is to be sensed via a resistor disposed on the high side of the circuit.
Referring first to FIG. 1, a typical three-phase motor controller circuit 2 is shown, in which a motor controller IC 4, such as the IR2130 IC (by International Rectifier, the assignee of the present application), produces gate signals to appropriately control the switching of six power MOSFETs or IGBTs 6, 8, 12, 14, 16, 18 arranged in a respective half-bridge circuits for driving the three respective phase inputs 20, 22, 24 to the three-phase motor 26.
The current delivered to the motor can be measured by a resistor 28 on the low side in the return path, as shown in FIG. 1.
However, a current sense resistor located on the low side bus such as shown in FIG. 1 will not detect faults in which current flows only through the upper side IGBTs or MOSFETs, such as a ground fault at the output of the converter or an inadvertent external connection of a short-circuit from an ac output terminal to the motor frame. To detect these types of faults, or to measure current flow into the motor for closed loop or open loop motor control, a current sense resistor can be located either on the high side bus, as shown in FIG. 2 by reference numeral 30, or in the path between the power switching node and the motor, as shown by reference numeral 32 in FIG. 2.
A significant problem associated with sensing current with a resistor at either of the locations shown in FIG. 2, however, is that the voltage level at the current sense resistor is at a high voltage potential. However, the circuitry responsible for conditioning and processing this signal is often referenced to a static node, commonly a signal ground shared with a MOS gate driver IC, such as the IR 2130, which operates at a relatively low potential. This results in a relatively large "common mode displacement"; i.e., a relatively large voltage difference between the high side voltage and the low side measurement circuity. The common mode displacement must be removed or "rejected", so as to recover only the signal of interest with minimal distortion to the signal.
Various techniques utilizing full or partial electrical isolation of unwanted common mode influence are well known, for example, opto-isolation or magnetic coupling of the signal to the measurement circuit. The protocol for accomplishing this transfer of information may be analog or digital in nature; however, such methods often embrace drawbacks including cost, complexity, size and various types of distortion to the wanted signal. Where electrical isolation between signal source and destination references is not possible or not desirable, the following considerations would typically apply.
An ideal case for recovering the signal of interest exists where the signal either is large, or can be made large, when compared to the common mode displacement, and such displacement remains static, or is relatively free from noise or other disturbances. In such a case, several established techniques, such as direct analog subtraction of the common mode displacement, or utilization of a differential amplifier stage, may be employed. Typically, such methods may be realized using inexpensive operational amplifier circuits.
In some cases, however, the signal of interest may be small when compared to the common mode displacement between the signal reference point and the measurement reference point. In such cases, the signal may suffer distortion from a number of sources. As an example of one such source, if a differential amplifier is used to remove the common mode potential, distortion will result from limitations in common mode rejection performance of the amplifier. To an extent, such distortion may be countered by amplification of the signal at source and subsequent scaling at the measurement reference point; however, reduction in distortion may still be insufficient to render this method entirely practical.
In other cases, the common mode displacement may be time variable or subject to noise originating from various sources. For example, in a power switching application, noise resulting from fast transients in the system are often difficult to contain and typically couple to the common mode potential. In power circuits, the common mode displacement is often time variable and may be high in both frequency and slew rate. In these cases, the signal of interest may prove difficult to entirely separate from the common mode potential. Again, distortion of the recovered signal may arise from a number of sources. For example, if a differential amplifier is used, inherent bandwidth limitations may impede complete rejection of the common mode signal, resulting in "contamination" of the signal.
Accordingly, it would be desirable to provide an improved circuit for recovering a signal of interest which is small compared to the common mode displacement between the signal reference point and the measurement reference point, and, more specifically, to provide such a circuit for sensing current flowing through a high side resistor in a motor controller.